diff --git a/usr.bin/compress/zopen.c b/usr.bin/compress/zopen.c
--- a/usr.bin/compress/zopen.c
+++ b/usr.bin/compress/zopen.c
@@ -620,7 +620,8 @@
 	}
 
 	/* High order bits. */
-	gcode |= (*bp & rmask[bits]) << r_off;
+	if (bits > 0)
+		gcode |= (*bp & rmask[bits]) << r_off;
 	roffset += n_bits;
 
 	return (gcode);